For ice I melting pressure as a function of temperature:

$$\pi = 1- 0.626000 \times 10^6 (1- \theta^{-3}) + 0.197135 \times 10^6 (1- \theta^{21.2})$$ 

where

$\pi =$ (pressure in MPa)/0.000611657 MPa

$\theta =$ (temperature in kelvins)/273.16K

Source is [International Equation of the Pressure along the Melting and along the Sublimation Curve of Ordinary Water substance][1] (no paywall).

Check original source before using equation in case I made typo(s). 



  [1]: http://www.nist.gov/data/PDFfiles/jpcrd477.pdf